Elastic or acoustic waves are typically used for a long time to design devices like resonators or filters. A well known example is the quartz resonator using Bulk Acoustic Waves illustrated by way of example with reference to FIG. 1. The principle is to use the piezoelectric effect of quartz to transform electric energy to acoustic energy inside a quartz crystal. Then, a resonance mode is obtained at the frequency such that the thickness of quartz plate is equal to half a wavelength. These devices are typically used at relatively low frequencies (some kHz to some MHz).
At higher frequencies (GHz range), the same principle can be applied. Since the wavelength is in the range of some micrometers, the thickness of the piezoelectric material is now very thin. To be able to manufacture these devices, thin film deposition techniques have to be used. Typically, thin films are deposited on a wafer. To confine the acoustic energy inside the piezoelectric film, the film can be mechanically isolated of the substrate by air (membrane approach) or reflective layers can be inserted between the substrate and the film (Solidly Mounted Resonator) as shown on FIG. 2.
SAW devices use so-called interdigitated transducers at the surface of piezoelectric substrates as illustrated with reference to FIG. 3. By piezoelectric effect, elastic waves are generated at the surface of the substrate. For resonators, the transducer is placed between two gratings. When the period of the grating is close to half the acoustic wavelength, the waves reflected on the individual electrodes are in phase and the grating reflects all the energy.
In all the devices described before, the elastic energy is confined for one direction. For example, for the quartz resonator or the thin film resonator, the energy is confined in the thickness of the piezoelectric film. For the SAW resonator, the energy is confined between the two gratings in the direction perpendicular to the electrodes. But for all these devices, it is difficult to confine the energy in the other directions. For the quartz resonators or the thin film resonator, the energy is confined below the electrodes thanks to the mass of the electrodes but spurious modes are often observed. In addition, some acoustic energy is often radiated outside of the resonator resulting in losses. For the SAW device, the lower velocity in transducers and gratings can result in wave guiding the energy inside the active aperture, but again, this also results in several modes and very often some energy is radiated resulting in losses. For example, for SAW RF resonators using Leaky SAW on Lithium Tantalate, it is known that acoustic radiation occurs and generates loss. Such is described in [1] J. Koskela, J. V. Knuuttila, T. Makkonen, V. P. Plessky, M. M. Salomaa, “Acoustic Loss Mechanisms in Leaky SAW Resonators on Lithium Tantalate”, IEEE Trans. On UFFC, vol. 48, no6, November 2001, pp 1517-1526; and [2] O. Holmgren, T. Makkonen, J. V. Knuuttila, M. Kalo, V. P. Plessky, W. Steichen, “Side Radiation of Rayleigh Waves from Synchronous SAW Resonators”, IEEE Trans. On UFFC, vol 54, no 4, April 2007, pp 861-869.
For SAW filters on quartz, transverse modes occur often resulting in ripple in the pass band or spurious emissions in the rejection band. One technique to excite only one mode is to adapt the velocity profiles in such a way that a mode having a flat amplitude in the active region can exist. This mode is then matched to the excitation profile meaning that it will be (almost) the only one excited. This technique called “piston mode” can be applied for both the BAW devices, as described in [3] J. Kaitila, M. Ylilammi, J. Ella, and R. Aigner, “Spurious resonance free bulk acoustic wave resonators”, in Proc. IEEE Ultrasonics symp, 2003, pp. 84-87; and for SAW devices, as described in [4] SAW FILTER OPERABLE IN A PISTON MODE, U.S. application Ser. No. 11/863,479 filed Sep. 28, 2007.
Even though it is efficient to reduce the spurious modes, some energy may still radiate outside of the resonators. It is desirable to suppress both the spurious modes and the radiation loss. In certain conditions, a grating having a two dimensional (2D) array of periodically placed obstacles can reflect any incident elastic wave with any incidence angle in a given frequency band. This is referred to as a “phononic crystal” or “phononic band gap” effect. The principle is that for a given incidence angle, the waves reflected on obstacles are all in phase for a frequency for which the period for this angle is half the wavelength (or a multiple of half a wavelength). The reflection coefficient on the grating is close to one inside a frequency band proportional to the reflection coefficient on one individual reflective obstacle. When the incidence angle varies, the center of this frequency band moves. If the individual reflection coefficient is large enough, a frequency band can exist when the reflection on the grating is close to one for all incidence angles. These phononic crystals have the potentiality to be used in a lot of devices and are very actively studied. By way of example as described in [5] A. Khelif, A. Choujaa, S. Benchabane, V. Laude and B. Djafari-Rouhani, “GUIDING AND FILTERING ACOUSTIC WAVES IN A TWO-DIMENSIONAL PHONONIC CRYSTAL”, 2004 IEEE Ultrasonics symp. proc, pp 654-657, a description is provided for possible applications of these devices for low frequencies. By way of further example, the application of phononic crystals to SAW devices is described in [6] Sarah Benchabane et al., “Silicon Phononic Crystal for Surface Acoustic Waves”, 2005 IEEE Ultrasonics symp. proc. pp 922-925. In these above described disclosures, very strong reflectors are used like holes as elements of the crystal.